1. Field of the Invention
The present invention relates to a speaker for use in TVs, radios, personal computers, and audio equipments, and more particularly to a speaker which is with a lowest resonant frequency, is good for reproducing a bass sound, and is excellent in durability and reliability.
2. Description of the Related Art
A dynamic speaker using a conical diaphragm is structured such that a diaphragm is disposed inside a frame so as to be suspended by way of a surround half-rolled and a spider, both of which work as suspensions, so that a voice coil bobbin having a voice coil wound thereon and fixedly attached to the inner perimeter of the diaphragm can move up and down accurately inside a magnetic circuit. In this structure, as is well known, the spider working as a suspension has a large influence on a lowest resonant frequency (fo) of a speaker, and the spider must be securely supported sideways. And, conventionally, the spider, which is disposed at the bottom part of the frame, has a smaller outer diameter smaller than the surround, which is disposed at the upper most part of the frame. In view of this fact, following speakers have been proposed.
First, a speaker has been proposed, in which the outer diameter of the diaphragm is set to be larger than the inner diameter of the surround shaped like a doughnut and consisting of a half-rolled portion, an outer edge (hereinafter referred to as “surround edge” as appropriate) and inner edge, the outer diameter of the spider is set to be larger than the outer diameter of the surround, and in which the diaphragm has its weight balanced due to the support at the inner diameter of the surround, thus preventing the diaphragm from rolling, and resulting in improved input. And, in the above proposed speaker, the outer diameter of the spider dimension can be increased without increasing the frame dimension in the diameter direction, thereby lowering the lowest resonant frequency (fo).
Second, a speaker has been proposed, in which the outer perimeter of the diaphragm is firmly held to the frame via the surround, either the inner peripheral end of the diaphragm or the voice coil bobbin is firmly held to the frame via the spider, cutaways are made at a portion which is formed at the upper most part of the frame and which is adapted to receive the surround edge (the portion to receive the surround edge is referred to as “surround edge receiving portion”) thereby allowing the spider to get clear of the portion into the frame without its outer diameter decreased, enabling the outer diameter of the spider to be set to be larger than the outer diameter of the surround, and keeping or making the lowest resonant frequency (fo) as low as possible, and in which the portions of the surround edge corresponding to the cutaways are supported by a gasket or cardboard to be placed thereon
The above described two speakers, in both of which the outer diameter of the spider is set to be larger than at least the outer diameter of the surround thereby making the lowest resonant frequency (fo) as low as possible, involve the following disadvantages.
In the first speaker, since the outer diameter. of the diaphragm is larger than the inner diameter of the surround, the diaphragm has to be uniquely configured such that the lower face of the outer periphery of the diaphragm is fixedly attached to the inner edge of the surround, and also since the entire weight balance of the diaphragm must be properly maintained, the production of the diaphragm is complicated. Further, the outer diameter of the yoke must be extended as far as to the joint with the frame. These factors will push up production cost.
On the other hand, in the second speaker, which is superior in that the configuration is not unique, since the cutaways must be made on the frame, the frame must be formed of a steel plate enabling the cutaways to be punched. As the steel plate has a small internal loss, the frame and the magnetic circuit must have butyl rubber or the like bonded on their surfaces so as to absorb the resonance generated by the diaphragm vibrating, which increases the number of constituent components thereby leading to increased cost.
Also, in the second speaker, the cutaways, which can be made on a frame shaped rectangular or elliptic, cannot be made when the frame is shaped circular. Further, in this speaker, since the outer edge of the surround has no support or protection provided on its lower face while having the gasket fixedly provided on its upper face, when the diaphragm moves downward considerably with a large input applied to the speaker, the surround including its outer edge is moved together downward considerably thereby possibly separating the surround edge from the gasket, which raises a problem with reliability.